1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine.
2. Description of the Related Art
Exhaust gas purification devices utilizing both a three-way catalyst and a NO.sub.X occluding and reducing catalyst are known in the art. A three-way catalyst is capable of purifying HC, CO and NO.sub.X at the same time when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is near a stoichiometric air-fuel ratio. A NO.sub.X occluding and reducing catalyst (herein after referred to as "an NORC") absorbs NO.sub.X in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NORC is lean, and releases the absorbed NO.sub.X and reduces the released NO.sub.X and NO.sub.X in the exhaust gas flowing into the NORC when the air-fuel ratio of the exhaust gas flowing into the NORC is a stoichiometric or a rich air-fuel ratio.
In this specification, the term "air-fuel ratio of the exhaust gas" means the ratio of the amounts of air and fuel supplied to the engine and exhaust gas passage upstream of a considered point. Therefore, if air or fuel is not supplied to the exhaust gas passage, the air-fuel ratio of the exhaust gas agrees with an operating air-fuel ratio of the engine (i.e., air-fuel ratio of the combustion in the engine).
In this type of the exhaust gas purification device, when either the three-way catalyst or the NORC deteriorates, the performance of the exhaust gas purification device as a whole deteriorates. Therefore, it is important to evaluate the abilities of the three-way catalyst and the NORC in order to facilitate the replacement of deteriorated catalyst.
Heretofore, various methods are proposed to evaluate the abilities of the three-way catalyst and the NORC.
For example, Japanese Unexamined Patent Publication (Kokai) No. 5-133264 proposes an evaluating device for evaluating the degree of deterioration of a three-way catalyst. The device in the '264 publication utilizes two air-fuel ratio sensors disposed in the exhaust gas passage of an engine upstream and downstream of the three-way catalyst, and evaluates the degree of deterioration of the three-way catalyst based on the amount of the deviation of the output of the downstream air-fuel ratio sensor from the output of the upstream air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst changes from a lean air-fuel ratio to a rich air-fuel ratio.
Further, Japanese Unexamined Patent Publication (Kokai) No. 8-232644 discloses an exhaust gas purification device which evaluates the deterioration of a NORC. The device in the '644 publication utilizes upstream and downstream air-fuel ratio sensors disposed in the exhaust gas passage of an engine at the inlet and outlet of the NORC and evaluates the deterioration of the NORC based on the time lapsed when the downstream air-fuel ratio sensor outputs a signal corresponding to a rich air-fuel ratio since the air-fuel ratio of the exhaust gas flowing into has changed from a lean air-fuel ratio to a rich air-fuel ratio.
However, when both of the abilities of a three-way catalyst and an NORC are evaluated by the devices in the above-explained publications, problems will occur.
For example, the devices of the '264 publication and the '644 publication require two air-fuel ratio sensors disposed in the exhaust gas passage upstream and downstream of the respective catalyst. Therefore, a total of three air-fuel ratio sensors are required for evaluating both of the abilities of the three-way catalyst and the NORC. This increases the manufacturing cost of the device due to an increase in the number of the air-fuel ratio sensors.
Further, the devices in the '264 publication and the '644 publication evaluate the abilities of catalysts having difference functions (i.e., the three-way catalyst and the NORC, respectively) and the methods used for the evaluation of the abilities of the three-way catalyst and the NORC are different from each other. Therefore, even if air-fuel ratio sensors are provided in the exhaust gas passage upstream and downstream of the respective catalysts, the evaluating operations of the catalysts must be performed separately, i.e., two evaluating operation must be performed for evaluating the abilities of the three-way catalyst and the NORC. During the evaluating operations of the catalysts, the engine is operated at an air-fuel ratio different from that in the normal operation of the engine and this air-fuel ratio during the evaluating operation is not necessarily optimum from the viewpoint of the exhaust emission or the performance of the engine. Therefore, when the separate evaluating operations for the three-way catalyst and the NORC are required, the frequency of the worsening of the performance and the emission of the engine increases.